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Stray readings on my XDM 1041 Multi Meter

FreeBird

I just want to confirm my suspicions.

I recently read an article by Forrest Mims regarding using LED's as light detectors.  I decided to see how my cheap chinese LED's would react so I plugged 2 wires into 2 adjacent rows on one of my bread boards.  Connected my DMM to them.  I then selected 1 of each color LED from my kit which contains 5 different colors (Red, Blue, Yellow, White and Green).  I noticed however, that I was getting stray voltage readings with nothing connected in the bread board.  I decided to record a series of 500 readings to see what the min, max and average of them would be.  This is what I got:

Min -0.0003
Max 0.0001
Average

-0.000025

I suspect, but would like someone smarter than me to confirm, that this is stray EMF in my lab.  I have a window AC unit running, my computer, a florescent light, etc.  Am I correct in my suspicion?  How might I confirm this, other than asking that is?

Thanks 

Charlie

  • Hi,

    An ideal voltmeter will have infinite impedance, so technically even static charges will cause a non-zero measurement, there's no need for it to be electromagnetic interference. In fact, multimeters do not make good EMI measurement devices at all.

    It's very normal for most multimeters to display a value when not connected to a circuit, or not shorted. If you short the two test leads, you'll see the measurement become zero (or extremely close to zero).

    To demonstrate that it is static charges, you could try rubbing a balloon with hair, and then hover one lead close to the balloon and gently move it around hovered over the surface, because those static charges will dissipate since the multimeter doesn't quite have infinite resistance. I've not tried this experiment, but I suspect you'll see non-zero measurements. Anyway, do report back the results, it's an interesting question.

  • It's possible that the 0.2mV fluctuations you are seeing is due to a limitation of the instrument's accuracy or environmental noise as you surmised.  Regarding your original aim to use an LED to detect light... what magnitude of voltages are you expecting?

  • in reply to shabaz

    Curiosity got the better of me, I decided to get off my backside and retrieve the multimeter. Rubbing a piece of plastic onto t-shirt (I used a ruler) and placing the multimeter negative lead near it, causes a very noticeable positive measurement (lower than a volt though, since the multimeter impedance isn't infinite). Can hold the lead near the plastic, and continually rub the plastic on cloth, and the reading is sustained. The positive lead was going nowhere, it was just left alone nearby.

  • in reply to beacon_dave

    Thanks.  Those were interesting blogs.

  • in reply to dang74

    I probably should have posted the results in the original message, but here it is.

    Blue LED(Min, Max, Avg) w/led flashlight about .75" away.  They were all near these values.

    0.5563
    0.7527

    0.665476

  •  Yes and no.  For minor fluctuations and tiny readings, I call this "meter magic" when teaching electricity and electronics.  As  has stated, it exists in every multimeter.  Not that I'm correct, but I attribute it to stray microcurrents in the sensing circuit.  Think of the precision needed at the input of an op amp not to have some error.  It is a differential comparison that is trying to obtain microvolt level accuracy.  To figure out your approximate noise floor, take your meter into an open field and see what kind of readings you get.  Each meter is going to be slightly different.
    I teach that meter magic readings indicate an open circuit - since we're not in the EMF business.

    On the "yes" front, hold one meter lead near a wall wart and the other away from it.  Vary the distance of the first meter lead from the wall wart.  Watch the reading change.  On my Fluke 87, I can hold one of the probes in the vicinity of a fluorescent light (or really cheap LED bulb) and get a consistent frequency reading even though the voltage still fluctuates.  You can try this with anything that you think might be generating EMF.  Just remember that the field strength falls off significantly with distance since the noise radiates in all directions (fills the sphere).

    Based on your averaged readings, I'd say that your input offset is finely tuned and you have a quality bench meter there.

  •  I remember reading about his LED experiments and his fights with AT&T(?) and the fascinating light and temperature data he collected from the airline industry being shutdown after 9/11. 

  • This diagram is copied from Wikipedia.  Imagine meter leads as the choke and capacitor and antenna. The LED is the diode. 

    The LED can work backwards a wee bit too, and the DVM itself may have some small noise inside as well. 

     image

  • in reply to kmikemoo

    I will try your experiments, possibly at 1 or 2 am this morning, if I wake up and can't get back to sleep. Otherwise at a more civilized time tomorrow morning like 8 or 9.  I will post the results.  Thank you